Growth in elevated CO(2) can both increase and decrease photochemistry and photoinhibition of photosynthesis in a predictable manner. Dactylis glomerata grown in two levels of nitrogen nutrition

Abstract

Biochemically based models of C(3) photosynthesis can be used to predict that when photosynthesis is limited by the amount of Rubisco, increasing atmospheric CO(2) partial pressure (pCO(2)) will increase light-saturated linear electron flow through photosystem II (J(t)). This is because the stimulation of electron flow to the photosynthetic carbon reduction cycle (J(c)) will be greater than the competitive suppression of electron flow to the photorespiratory carbon oxidation cycle (J(o)). Where elevated pCO(2) increases J(t), then the ratio of absorbed energy dissipated photochemically to that dissipated non-photochemically will rise. These predictions were tested on Dactylis glomerata grown in fully controlled environments, at either ambient (35 Pa) or elevated (65 Pa) pCO(2), and at two levels of nitrogen nutrition. As was predicted, for D. glomerata grown in high nitrogen, J(t) was significantly higher in plants grown and measured at elevated pCO(2) than for plants grown and measured at ambient pCO(2). This was due to a significant increase in J(c) exceeding any suppression of J(o). This increase in photochemistry at elevated pCO(2) protected against photoinhibition at high light. For plants grown at low nitrogen, J(t) was significantly lower in plants grown and measured at elevated pCO(2) than for plants grown and measured at ambient pCO(2). Elevated pCO(2) again suppressed J(o); however growth in elevated pCO(2) resulted in an acclimatory decrease in leaf Rubisco content that removed any stimulation of J(c). Consistent with decreased photochemistry, for leaves grown at low nitrogen, the recovery from a 3-h photoinhibitory treatment was slower at elevated pCO(2).

Figure 1

Light-saturated photosynthesis: high nitrogen. a, The responses of light-saturated CO₂ uptake (A_sat) against intercellular CO₂ concentration (C_i) for leaves of D. glomerata grown in high nitrogen and at either elevated (black symbols and black lines) or current ambient (white symbols and dotted lines) pCO₂. Values of V_c,max and J_max, calculated using the equations and constants in von Caemmerer (2000) and Bernacchi et al. (2001), were used to fit a nonlinear regression to observed values above (J_max) and below (V_{c, max}) the inflection of the curves. Also shown are the supply functions for each curve (dashed line) that indicate the operating point of photosynthesis at the growth pCO₂ for each treatment. Data points shown are the means (±1 se) for five replicate leaves. Measurements were made in 21 kPa O₂ and at a PPFD of 1,300 μmol m⁻² s⁻¹. b, J_t, J_c, and J_o for ambient (white bar) and elevated (black bar) pCO₂ treatments were calculated for measurements at the respective growth pCO₂ for each group of leaves using the equations of Valentini et al. (1995). Values shown are the means (±1 se) for five replicate leaves.

Figure 2

Light-saturated photosynthesis: low nitrogen. a, Plot of light-saturated A against C_i for leaves of D. glomerata grown in low nitrogen. As described previously for Figure 1. b, Measurements of J_t, J_c, and J_o made at the respective growth pCO₂ for D. glomerata grown in low nitrogen. As described previously for Figure 1.

Figure 3

Photoinhibition and recovery: high nitrogen. a, Photo-inhibitory reduction in F_v/F_m. b, Changes in F_o during a 3-h exposure to a PPFD of 2,000 μmol m⁻² s⁻¹. c, Recovery of F_v/F_m measured after 10 min dark adaption (black lines), and F_v'/F_m' measured under growth PPFD (dashed lines), for D. glomerata grown in high nitrogen. Plants were grown, photoinhibited, then allowed to recover in their growth pCO₂ either ambient (white symbols) or elevated (black symbols). Each symbol represents the mean (±1 se) for 10 replicate plants.

Figure 4

Photoinhibition and recovery: low nitrogen. a, Photo-inhibitory reduction in F_v/F_m. b, Changes in F_o during a 3-h exposure to a PPFD of 2,000 μmol m⁻² s⁻¹. c, Recovery of F_v/F_m measured after 10 min of dark adaption (black line), and F_v'/F_m' measured under growth PPFD (dashed line), for D. glomerata grown in low nitrogen. As described previously for Figure 3.